Catalyst composition and method for use in selective catalytic reduction of nitrogen oxides

ABSTRACT

Catalyst composition for selective reduction of nitrogen oxides and soot oxidation comprising a physical mixture of one or more acidic zeolite or zeo-type components with one ore more redox active metal compounds and a method for selective reduction of nitrogen oxides and soot oxidation by use of the catalyst composition.

The present invention relates to catalyst composition for use inselective reduction of nitrogen oxides in off-gases by reaction withammonia or a precursor thereof.

Catalysts for NH₃—SCR, i.e. selective reduction of nitrogen oxides (NOx)by use of ammonia as reductant are well known in the art. Thosecatalysts include zeolitic material, optionally promoted with copper oriron

The problem to be solved by this invention is to provide a catalystcomposition and method for the reduction of nitrogen oxides with aDeNO_(x) activity at reaction temperatures between 150 and 550° C.

Off-gases from lean combustion engines contain in addition to NOx,hydrocarbons, CO and soot particles which can be reduced or removed bycatalytic oxidation. Consequently, the catalyst composition and methodof this invention shall further include soot and hydrocarbon oxidationactivity simultaneously with the DeNOx activity.

Our recent studies revealed several examples of a pronounced synergisticeffect in composite catalysts prepared by mechanical mixing of acidiczeolite or zeotype powder and redox active metal compounds.

We have found that catalyst composition comprising a one or more acidiczeolite or zeotype components physically admixed with one ore more redoxactive metal compounds shown an improved activity in the selectivereduction of nitrogen oxides and oxidation of hydrocarbons, CO and sootcontained in off-gas.

The term “redox active metal compounds” as used herein relates to metalcompounds which reversibly can be oxidized and reduced in terms ofchanges in oxidation number, or oxidation state, of the metal atom orcompound.

Pursuant to the above findings, the present invention provides acatalyst composition for selective reduction of nitrogen oxides and sootoxidation comprising one or more acidic zeolite or zeotype componentsselected from the group consisting of BEA, MFI, FAU, FER, CHA, MOR ormixtures thereof physically admixed with one ore more redox active metalcompounds selected from the group consisting of Cu/Al₂O₃, Mn/Al₂O₃,CeO₂—ZrO₂, Ce—Mn/Al₂O₃ and mixtures thereof.

Catalyst compositions prepared by mechanical mixing of the abovementioned zeolites or zeotype materials and redox metal componentsmixing according to the invention exhibit a pronounced synergisticeffect. DeNO_(x) activity of such composite catalysts significantlyexceeds activity of their individual components.

The acidic zeolite or zeotype component can be used in protonic form orpromoted with Fe.

Preferably, the weight ratio between the zeolite components and theredox components is between 1:1 to 1:50

In an embodiment of the invention, the redox components are dispersed ona support selected from the group consisting of of Al₂O₃, TiO₂, SiO₂,CeO₂, ZrO₂ or mixtures thereof.

It is generally preferred that the mean molar ratio Si/Al of the zeolitecomponents according to the invention is from 5 to 100.

The above described catalyst composition according to the invention canbe utilised as coating material or as coat on structured bodies ofmetallic, ceramic, metal oxide, SiC or silica materials or fibres.

Thus, the invention provides furthermore a monolithic structured bodybeing coated with a catalyst composition according to anyone of theabove disclosed embodiments of the invention.

The monolithic structured body is preferably made from metallic,ceramic, metal oxide, SiC or silica fiber materials.

The monolithic structured body may be in form of a particle filter, e.g.a honeycomb structured filter or a wall flow filter.

In further an embodiment, the catalyst composition is coated on the bodyin of two or several separate catalyst layers in series or as two orseveral catalyst layers in parallel and wherein the layers havedifferent compositions or layer thicknesses.

Specific advantages resulting from the invention are

1) Addition of CeO₂—ZrO₂, Cu/Al₂O₃, Mn/Al₂O₃ or Ce—Mn/Al₂O₃ to acidiczeolite or zeotype in protonic form or promoted with iron markedlyenhances DeNO_(x) activity at T_(react)<250° C. without increasingamount of zeolite component. In this case, overall volume of thecatalyst is increased by the volume of redox component added.

2) Alternatively, amount of expensive zeolite/zeotype component in thecomposite catalyst can be significantly reduced by its replacement withequivalent volume of redox component. In this case overall volume of thecatalyst remains constant, but the amount of zeolite component can bedecreased by 2-5 times, without notable sacrificing DeNO_(x)performance. When Ce—Mn/Al₂O₃ component is used for the catalystpreparation, notable improvement of NOx conversion at T_(react)<250° C.is observed despite decreased amount of zeolite component.

3) In addition to favourable DeNO_(x) activity,[CeO₂—ZrO₂+zeolites/zeotypes] or [Ce—Mn/Al₂O₃+zeolites/zeotypes]compositions demonstrate significant soot oxidation activity, whichmakes them promising candidates for development of integratedDeNOx-DeSoot catalytic systems.

4) In addition to favorable DeNO_(x) activity,[CeO₂—ZrO₂+zeolites/zeotypes] or [Ce—Mn/Al₂O₃+zeolites/zeotypes]compositions demonstrate significantly lower ammonium slip at hightemperature due to selective oxidation of excess ammonia.

The invention provides additionally a method for the selective reductionof nitrogen oxides and oxidation of soot contained in an off-gascomprising the step of contacting the off-gas in presence of ammoniawith a catalyst composition comprising one or more acidic zeolite orzeotype components selected from the group consisting of BEA, MFI, FAU,FER, CHA, MOR or mixtures thereof physically admixed with one ore moreredox active metal compounds selected from the group consisting ofCu/Al₂O₃, Mn/Al₂O₃, CeO₂—ZrO₂, Ce—Mn/Al₂O₃ and mixtures thereof.

The acidic zeolite or zeotype component can be used in protonic form orpromoted with Fe

In an embodiment of the inventive method, the one or more redox activemetal compounds are dispersed on a support selected from the groupconsisting of Al₂O₃, TiO₂, SiO₂, ZrO₂ or mixtures thereof.

In still an embodiment of the inventive method, the catalyst compositionis contacted with the off-gas at a temperature below 250° C.

In a further embodiment of the inventive method excess of ammonia isselectively oxidized to nitrogen by contact with the catalystcomposition.

EXAMPLES Example 1

Synergistic effect in NH₃-DeNOx over CeO2-ZrO2+H-Beta zeolite catalystcompositions.

[CeO₂—ZrO₂+H-Beta zeolite] composite catalyst was prepared by thoroughmixing 74 wt % CeO₂-26 wt % ZrO₂ powder with H-Beta powder at a weightratio of 10. This weight ratio results in volume ratio of componentsCeO₂—ZrO₂/H-Beta=3/1 due to difference in densities of these materials.The powders were thoroughly grinded in agate mortar for 10-15 min,followed by pelletization. The pellets were crushed and sievedcollecting 0.2-0.4 mm fraction for catalytic test. Similarly pelletized74 wt % CeO₂-26 wt % ZrO₂, H-Beta, and Fe-Beta zeolite were used asreference samples.

The catalysts were tested in the NH₃-DeNOx in the temperature range of150-550° C. The test was performed under following conditions:decreasing reaction temperature with a rate of 2° C./min, feed gascomposition: 500 ppm NO, 540 ppm NH₃, 10 vol % O₂, 6 vol % H₂O, balancedwith N₂ to obtain a total flow of 300 mL/min.

Catalyst Loading and Resulted GHSV:

0.197 g with 74 wt % CeO₂—ZrO₂+0.02 g H-Beta zeolite, catalyst volume0.134 ml, GHSV=135 000 h⁻¹

Under these conditions CeO₂—ZrO₂+H-Beta zeolite composite catalystshowed DeNO_(x) activity, which substantially exceeded activities ofindividual 74 wt % CeO₂—ZrO₂ (0.131 g CeO₂—ZrO2, catalyst volume 0.067ml, GHSV=270,000 h⁻¹) and H-Beta zeolite (0.04 g, catalyst volume 0.067ml, GHSV=270 000 h−1), indicating pronounced synergistic effect betweencomponents of composite catalyst as shown in FIG. 1.

NO_(x) conversion over composite catalyst is similar to NO_(x)conversion over commercial Fe-Beta zeolite (Fe-Beta) at 230-550° C., andexceeds NO_(x) conversion over Fe-Beta zeolite at 150-200° C.

Example 2

Enhanced DeNOx performance of [CeO₂—ZrO₂+Fe-Beta] composite catalyst atT_(react)<250° C.

Two samples of [CeO₂—ZrO₂+Fe-Beta zeolite] composite catalyst wereprepared by thorough grinding of 74 wt % CeO₂—26 wt % ZrO₂ and Fe-Betazeolite powders.

A first sample was prepared by mixing 74 wt % CeO₂-26 wt % ZrO₂ andFe-Beta zeolite powders at a weight ratio of 3.3. This weight ratioresults in a volume ratio of 74 wt % CeO₂—26 wt % ZrO₂/Fe-Betacomponents in composite catalyst=1/1. A second sample was prepared bymixing 74 wt % CeO₂-26 wt % ZrO₂ and Fe-Beta powders at a weight ratioof 10. For the second sample volume ratio of 74 wt % CeO₂-26 wt %ZrO₂/Fe-Beta zeolite equals 3/1.

After grinding in agate mortar for 10-15 min, the resulted mixtures werepelletized. The pellets were crushed and sieved collecting 0.2-0.4 mmfraction for catalytic test. Similarly pelletized Fe-Beta zeolite wasused as reference.

Activities of the prepared samples were tested using the followingcatalyst loading which kept constant amount of Fe-Beta zeolite componentin the reactor:

The first sample with 1/1 volume component ratio: [0.065 g 74 %CeO₂—ZrO₂+0.02 g Fe-Beta zeolite].

The second sample with 3/1 volume component ratio: [0.197 g 74 %CeO₂—ZrO₂+0.02 g Fe-Beta zeolite].

Reference sample: 0.02 g Fe-Beta zeolite.

The catalysts were tested in NH₃-DeNO_(x) within the temperature rangeof 150-550° C. The test was performed under following conditions:decreasing reaction temperature with a rate of 2° C./min, feed gascomposition: 500 ppm NO, 540 ppm NH₃, 10 vol % O₂, 6 vol % H₂O, balancedwith N₂ to obtain a total flow of 300 mL/min.

Catalyst Loading and Resulted GHSV:

[0.197 g 74 % CeO₂—ZrO2+0.02 g Fe-Beta zeolite], catalyst vol.=0.134 ml,GHSV=135 000 h⁻¹;

[0.065 g 74 % CeO₂—ZrO2+0.02 g Fe-Beta zeolite], catalyst vol.=0.067 ml,GHSV=270 000 h⁻¹;

0.02 Fe-Beta zeolite, catalyst vol.=0.034 ml, GHSV=540 000 h⁻¹.

Under these test conditions [CeO₂—ZrO₂+Fe-Beta zeolite] compositecatalysts showed enhanced DeNO_(x) activity within low-temperature range(150-300° C.), which significantly exceeded activity of individualFe-Beta zeolite, as shown in FIG. 2. It is important to note that theactivity of [CeO₂—ZrO₂+Fe-Beta zeolite] is improved when the amount ofCeO₂—ZrO₂ component was increased.

Example 3

Catalyst with reduced amount of zeolite component.

Three samples of [CeO₂—ZrO₂+Fe-Beta zeolite] composite catalyst wereprepared by thorough grinding of 74 wt % CeO₂—26 wt % ZrO₂ powder withFe-Beta zeolite powder:

A first sample was prepared by mixing 74 wt % CeO₂-26 wt % ZrO₂ andFe-Beta powders at a weight ratio of 3.3. In this case volume ratio of74 wt % CeO₂-26 wt % ZrO₂/Fe-Beta zeolite equals 1/1.

A second sample was prepared by mixing 74 wt % CeO₂-26 wt % ZrO₂ andFe-Beta zeolite powders at a weight ratio of 15.5. For the second samplevolume ratio of 74 wt % CeO₂-26 wt % ZrO₂ and Fe-Beta zeolite componentsequals 5/1.

A third sample was prepared by was prepared by mixing 74 wt % CeO₂-26 wt% ZrO₂ and Fe-Beta zeolite powders at a weight ratio of 30. For thesecond sample volume ratio of 74 wt % CeO₂-26 wt % ZrO₂ and Fe-Betazeolite components equals 10/1.

After grinding in agate mortar for 10-15 min, the resulted mixtures werepelletized. The pellets were crushed and sieved collecting 0.2-0.4 mmfraction for catalytic test. Similarly pelletized Fe-Beta zeolite wasused as reference.

Activities of the prepared samples were tested using the followingcatalyst loading which kept constant volume of the catalyst in thereactor. In all experiments described below overall volume on thecatalyst loaded was 0.067 ml, which results in GHSW˜270 000 h⁻¹:

First sample (1/1 vol component ratio): [0.065 g 74 wt % CeO₂—ZrO₂+0.02g Fe-Beta zeolite].

Second sample (5/1 vol component ratio): [0.109 g 74 wt %CeO₂—ZrO₂+0.007 g Fe-Beta zeolite].

Third sample (10/1 vol component ratio): [0.119 g 74 wt %CeO₂—ZrO₂+0.0035 g Fe-Beta zeolite].

Reference sample: 0.02 g Fe beta-zeolite.

Feed gas composition: 540 ppm NH₃, 500 ppm NO, 10% O₂, 6% H₂O balancewith N₂.

Under these conditions [CeO₂—ZrO₂+Fe-Beta zeolite] composite catalystsshowed DeNO_(x) performances, which were essentially identical to theperformance of reference Fe-Beta zeolite sample, despite significantlyreduced amount of zeolite catalyst (Fe-Beta zeolite) loaded into thereactor as a part of composite [CeO₂—ZrO₂+Fe-Beta zeolite].

The data in FIG. 3 show that amount of zeolite can be reduced at least10 times without sacrificing DeNO_(x) performance of [CeO₂—ZrO₂+Fe-Betazeolite] by its replacement with corresponding volume of CeO₂—ZrO₂.

Example 4

Enhanced DeNO_(x) performance of [Ce—Mn/Al₂O₃+Fe-Beta zeolite] compositecatalyst at T_(react)<250° C.

[Ce—Mn/Al₂O₃+Fe-Beta] composite catalysts were prepared by thoroughmixing 15 wt % Ce-15 wt % Mn/Al₂O₃ powder with Fe-Beta powder at aweight ratio of 0.8:1; 1.7:1 and 3.4:1 keeping the same total volume ofthe catalyst constant. These weight ratios result in volume ratio ofcomponents Ce—Mn/Al₂O₃/Fe-Beta=2/1; 1/1 and 1/2 due to difference indensities of these materials. The powders were thoroughly grinded inagate mortar for 10-15 min, followed by pelletization. The pellets werecrushed and sieved collecting 0.2-0.4 mm fraction for catalytic test.Similarly pelletized Fe-Beta was used as reference.

The catalysts were tested in the NH₃-DeNOx in the temperature range of150-550° C. The test was performed under following conditions:decreasing reaction temperature with a rate of 2° C./min, feed gascomposition: 500 ppm NO, 540 ppm NH₃, 10 vol % O₂, 6 vol % H₂O, balancedwith N₂ to obtain a total flow of 300 mL/min.

Catalyst Load:

0.04 g Fe-Beta and [0.045 g Ce—Mn/Al₂O₃+0.013 g Fe-Beta] (2/1 ratio),[0.034 g Ce—Mn/Al₂O₃+0.02 g Fe-Beta] (1/1 ratio), [0.022 gCe—Mn/Al₂O₃+0.027 g Fe-Beta] (1/2 ratio).

Under these conditions all [Ce—Mn/Al₂O₃+Fe-Beta] composite catalystsshowed DeNO_(x) activity, which radically exceeded activities ofindividual Ce—Mn/Al₂O₃ and Fe-Beta at temperatures below 350° C.,indicating pronounced synergistic effect between components of compositecatalyst (FIG. 4). Besides that, ammonia slip on composite catalysts wassignificantly lower than for a reference Fe-Beta catalyst indicatingthat those composite systems can be used as integrated DeNOx-ASC.

Example 5

Enhanced DeNOx performance of [10 wt % Cu/Al₂O₃+H-zeolite] compositecatalysts.

Three samples of [10 wt % Cu/Al₂O₃+H-zeolite] composite catalyst wereprepared by thorough grinding of 10 wt % Cu/Al₂O₃ and H-Beta, H-ZSM-5,or H-ferrierite powder.

A first sample was prepared by mixing 10 wt % Cu/Al₂O₃ and H-Beta(Si/Al=20) powders at a weight ratio of 1/1.

A second sample was prepared by mixing 10 wt % Cu/Al₂O₃ and H-ZSM-5powders (Si/Al=20) at a weight ratio of 1/1.

A third sample was prepared by mixing 10 wt % Cu/Al₂O₃ and H-ferrieritepowders (Si/Al=32) at a weight ratio of 1/1.

After grinding in agate mortar for 10-15 min, the resulted mixtures werepelletized. The pellets were crushed and sieved collecting 0.2-0.4 mmfraction for catalytic test.

Similarly corresponding pelletized zeolites (H-Beta, H-ZSM-5, andH-ferrierite) were used as reference.

Activities of the prepared samples were tested using the followingcatalyst loading which kept constant amount of zeolite component in thereactor:

The first sample with 1/1 weight component ratio: [0.040 g 10 wt %Cu/Al₂O₃+0.040 g H-Beta].

The second sample with 1/1 weight component ratio: [0.040 g 10 wt %Cu/Al₂O₃+0.040 g H-ZSM-5].

The third sample with 1/1 weight component ratio: [0.040 g 10 wt %Cu/Al₂O₃+0.040 g H-ferrierite].

Reference samples: 0.040 g H-Beta; 0.040 g H-ZSM-5, or H-ferrierite, or0.040 g 10 wt % Cu/Al₂O₃.

The catalysts were tested in NH₃-DeNO_(x) within the temperature rangeof 150-550° C. The test was performed under following conditions:decreasing reaction temperature with a rate of 2° C./min, feed gascomposition: 500 ppm NO, 540 ppm NH₃, 10 vol % O₂, 6 vol % H₂O, balancedwith N₂ to obtain a total flow of 300 mL/min.

Catalyst Loading and Resulted GHSV:

[0.040 g 10 wt % Cu/Al₂O₃+0.040 g H-Beta], catalyst vol.=0.134 ml,GHSV=135 000 h⁻¹;

[0.040 g 10 wt % Cu/Al₂O₃+0.040 g H-ZSM-5], catalyst vol.=0.134 ml,GHSV=135 000 h⁻¹;

[0.040 g 10 wt % Cu/Al₂O₃+0.040 g H-ferrierite], catalyst vol.=0.134 ml,GHSV=135 000 h⁻¹;

Reference Catalysts

0.040 g H-Beta, catalyst vol.=0.067 ml, GHSV=270,000 h⁻¹;

0.040 g H-ZSM-5, catalyst vol.=0.067 ml, GHSV=270,000 h⁻¹;

0.040 g H-ferrierite, catalyst vol.=0.067 ml, GHSV=270,000 h⁻¹;

0.040 g Cu/Al₂O₃, catalyst vol.=0.067 ml, GHSV=270,000 h⁻¹.

Under these test conditions [10 wt % Cu/Al₂O₃+H-zeolite] compositecatalysts showed enhanced DeNO_(x) within the whole temperature range(150-550° C.), which significantly exceeded activity of individualcomponents, as shown by comparing FIG. 5 and FIG. 6.

Example 6

Catalyst with enhanced soot oxidation activity.

[CeO₂—ZrO₂+Fe-Beta] with 3/1 vol. component ratio was prepared asdescribed in Example 2. For testing soot oxidation activity of[CeO₂—ZrO₂+Fe-Beta] a part of pelletized sample was crushed, and thecatalyst powder was mixed with soot (“Printex U”, Degussa) at a weightratio catalyst/soot=1/10. Soot and catalyst were mixed by shaking in aglass bottle for 5 min, thus establishing loose contact between soot andthe catalyst. Reference sample was prepared in a similar manner usingFe-Beta powder.

Soot oxidation was carried out at temperature ramp=10° C./min in a flowof dried air. Profiles of soot oxidation over [CeO₂—ZrO₂+Fe-Beta] andFe-Beta are displayed in FIG. 7. [CeO₂—ZrO₂+Fe-Beta] significantlyhigher activity in soot oxidation then individual Fe-Beta, as evidencedby a shift of soot oxidation maximum from ˜600° C. for (Fe-Beta+soot) to˜420° C. for ([CeO₂—ZrO₂+Fe-Beta]+soot).

1. Catalyst composition for selective reduction of nitrogen oxides andsoot oxidation comprising of one or more acidic zeolite or zeotypecomponents selected from the group consisting of BEA, MFI, FAU, FER,CHA, MOR or mixtures thereof physically admixed with one ore more redoxactive metal compounds selected from the group consisting of CU/Al₂O₃,Mn/Al₂O₃, Ce0 ₂—ZrO₂, Ce—Mn/Al₂O₃ and mixtures thereof.
 2. The catalystcomposition of claim 1, wherein weight ratio between the zeolitecomponents and the redox components is between 1:1 and 1:50.
 3. Thecatalyst composition of claim 1, wherein the one or more redox activemetal compounds are dispersed on a support selected from the groupconsisting of of A1 ₂ 0 ₃, Ti0 ₂, Si0 ₂, Zr0 ₂ or mixtures thereof. 4.The catalyst composition according to claim 1, wherein the one or moreacidic zeolite or zeotype components are in protonic form or promotedwith Fe.
 5. The catalyst composition according to claim 1, wherein meanmolar ratio of Si/Al of the one or more acidic zeolite or zeotypecomponents is from 5 to
 100. 6. The catalyst composition according toclaim 1, wherein the one or more acidic zeolite or zeotype componentsare selected from the group consisting of beta-zeolite, ZSM-5 andferrierite.
 7. A monolithic structured body being coated with a catalystcomposition according to claim
 1. 8. The monolithic structured body ofclaim 7, wherein the monolithic structured body is in a form of aparticle filter.
 9. The monolithic structured body of claim 7, whereinthe catalyst composition is coated on the body in two or severalseparate catalyst layers in series or as two or several catalyst layersin parallel and wherein the layers have different compositions or layerthicknesses.
 10. Method for the selective reduction of nitrogen oxidesand oxidation of soot contained in an off-gas comprising the step ofcontacting the off-gas in presence of ammonia with a catalystcomposition comprising one or more acidic zeolite or zeotype componentsselected from the group consisting of BEA, MFI, FAU, FER, CHA, MOR ormixtures thereof physically admixed with one ore more redox active metalcompounds selected from the group consisting of CU/AI₂O₃, Mn/Al₂O₃,CeO₂—ZrO₂, Ce—Mn/Al₂O₃ and mixtures thereof.
 11. The method accordingclaim 10, wherein the one or more redox active metal componentsdispersed on the surface of the one or more zeolite components containCe, Mn, Zr, Cr or mixtures thereof.
 12. The method of according to claim10, wherein the catalyst composition is contacted with the off-gas at atemperature below 250° C.
 13. The method according to claim 10, whereinexcess of ammonia is selectively oxidized to nitrogen by contact withthe catalyst composition.
 14. The method according to claim 10, whereinthe one or more acidic zeolite or zeotype components are in protonicform or promoted with Fe.
 15. The method according to claim 10, whereinmean molar ratio of Si/Al of the one or more acidic zeolite or zeotypecomponents is from 5 to
 100. 16. The method according to claim 10,wherein the one or more acidic zeolite or zeotype components areselected from the group consisting of beta-zeolite, ZSM-5 andferrierite.